The present invention relates to a broadcast receiver and to a circuit arrangement for applying a tuning voltage to a tuning device of a broadcast receiver.
It is known in the field of radio and television receivers to use devices which vary their capacitance according to an applied control voltage, as in the case of varactor or "varicap" diodes, for example, to enable tuning to be effected by varying the control voltage. In practice, the variations in the capacitance of the varactor diodes alter the frequency of the local oscillator of the receiver which changes the received radio frequency signal down to the intermediate frequency of the receiver.
In the case of television signals, the receiver must be capable of receiving a wide range of transmission channels on two different frequency bands, namely VHF and UHF.
However, these two bands are not adjacent. In fact, the first covers a frequency range of 47-300 MHz and the second 470-900 MHz.
As it is not simple, with a single local oscillator using varactor diodes, to obtain a tuning range wide enough to cover both the VHF and UHF bands, the normal practice is to use two independent oscillators. In this case, there is only one arrangement for generating the tuning voltage which is connected selectively to one or the other of the oscillators, which is activated depending on whether the channel to be received is in the VHF or UHF band.
Systems are also known in which, by means of a double frequency conversion performed inside the tuner, a single oscillator can be used, piloted by a single tuning voltage generating arrangement, which can cover the whole frequency range required for tuning channels both on the VHF and UHF bands. However, as these channel bands are not adjacent when switching from a VHF to a UHF channel, other types of broadcasting channels will be received, such as those reserved for police broadcasts, to which it is illegal to listen. Provision must therefore be made to prevent the receiver from tuning into these channels.
Apart from the legal aspect, there are also other disadvantages. One of these is disturbance on the screen in the case of a television receiver capable of receiving radio broadcasts. As coding of these is completely different from television broadcastings, radio broadcasts received on a television receiver result in a series of moving black lines which may be interpreted by the user as a breakdown of the receiver.